1. Field of the Invention
The invention concerns a circuit arrangement for firing an electric valve, particularly a thyristor, with a pulse capacitor which can be charged from the a-c voltage at the anode-cathode path of the electric valve via an external resistor and can be discharged via a contactless switching device for firing the electric valve via its control path.
2. Description of the Prior Art
A circuit arrangement of this kind is known from the German Pat. No. 1,538,099. The circuit of this German Patent is used particularly for firing one or more thyristors which are arranged in a rectifier circuit for large currents and high voltages. In the known circuit arrangement, the electric valve to be fired is shunted by a series circuit consisting of an external resistor, a charging diode and a pulse capacitor. The charging diode is poled in the current-flow direction of the electric valve. A blocking diode is further arranged antiparallel to the series circuit consisting of the charging diode and the pulse capacitor. The pulse capacitor is charged by way of the external resistor, the external capacitor and the charging diode from the reverse voltage at the anode-cathode path of the electric valve. The charged pulse capacitor can discharge to the control path of the electric valve by way of a contactless switching device, which may be, for instance, a light-controlled thyristor or a thyristor externally controlled via a control transformer. The pulse capacitor is generally shunted, furthermore, by a voltage-limiting element, e.g., a Zener diode.
In the known circuit arrangement, the firing power required at the control path of the electric valve for firing the same is obtained from the a-c voltage at its anode-cathode path. In the interest of reliable firing, the firing power should be chosen high. In the known circuit arrangement, this requirement can be met by making the capacity of the external capacitor and the pulse capacitor large. Such an increase in the capacity, however, frequently is a not advantageous for the circuit, particularly for a rectifier circuit, in which the controlled valve in question is arranged.
It is an object of the present invention to provide a firing circuit such that the firing power available for firing the electric valve is increased without the possibility that the increased capacity exerts a detrimental influence on the circuit in which the electric valve is arranged.